The present invention generally relates to noise reduction circuits for video signals, and more particularly to a noise reduction circuit which can satisfactorily reduce a noise component in a video signal without introducing undesirable effects to a video signal component by using correlations in the video signal such as a correlation within a line, a correlation between lines, and a correlation between fields.
Conventionally, a noise component within a video signal is reduced in noise reduction circuits which use correlations such as a correlation within a line (hereinafter simply referred to as a within-line correlation), a correlation between lines (hereinafter simply referred to as a line correlation), and a correlation between fields (hereinafter simply referred to as a field correlation). The construction of the conventional noise reduction circuits will be described later on in the specification by referring to drawings. However, the conventional noise reduction circuits are constructed independently depending on the kind of correlation which is taken into account to reduce the noise component. Further, especially in the case where the video signal has no within-line correlation, no line correlation, and no field correlation, the pattern of the reproduced picture becomes deteriorated when the video signal is passed through a noise reduction circuit which uses one of the correlations in the video signal to reduce the noise component. For this reason, each of the conventional noise reduction circuits comprises an internal limiter circuit having a limiting level thereof selected to a low value to such an extent that no undesirable effects are introduced to the video signal. Accordingly, the conventional noise reduction circuits had a disadvantage in that a satisfactory noise reducing effect cannot be obtained. On the other hand, when the limiting level is increased to improve the noise reducing effect so that the signal-to-noise (S/N) ratio is improved, there is a problem in that the pattern of the reproduced picture becomes faded in the case where no correlation exists in the information contents of the video signal.